I had this same problem with Ubuntu, I have sound for all other applications, no sound in firefox. The music appears to be streaming but I am getting no sound. my computer is hooked up to my home home theater via USB. Amarok would not work at first, I fixed this by changing the output plugin to alsa. So how can I do this for firefox or configure Mplayer.

I can listen to sound through any other progam, Amarok, Totem, System beeps, Rythmbox all work.Everything did out of the box, except Amarok which I easily fixed by changing the output plugin to alsa.This is only one PC, my home stereo is a JVC that simply has a USB input to receive audio.

just to note, Mozilla songbird 0.5 worked 100% out of box. ??? back to firefox, changing Mplayer's output plugin to alsa does not work. many different configuration's possible, tried most none work. does anyone have any suggestions.